High-temperature stainless steel



vFeb. 6, 1951 l -w,--cv:. LA'Rl-(EJR l HIGH-TEMPERATURE-STA1NLESS STEEL: v

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Patented Feb. 6, 1951 2,540,509 HIGH-TEMPERATURE STAINLESS STEEL William Charles Clarke, Jr.. Dundalk, Md., assignor to Armco Steel Corporation, a corporation of Ohio Application October 14, 1947, Serial No. 779,665

l Claims.

The present application is a continuation-inf part of my copending applications, Serial No. 645,015, filed February l, 1946, now Patent 2,447,896, and Serial No. 671,905, filed May 23, 1946, now Patent 2,447,897, both of August 24, 1948, and the invention relates to high temperature steel.

An object of my invention is the-provision of strong, durable and reliable high temperature stainless steel, and products and articles of the steel, which are capable of resisting the development of sigma phase while under load at high temperatures, which are resistant to stress-rupture and creep at the high temperatures encountered under load, and which are well suited -ior resisting attack by hot corrosive matter and avoiding the formation of heat scale.

A further object of my invention is the provision of high temperature stainless steel of the character indicated which has good hot working properties.

A still further object is the provision of stainless steel of the character indicated, heat treated for enhanced high temperature properties.

Other objects of the invention in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of ingredients, and in the articles, products and manufactures thereof, also in the several method steps and the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

The single ligure of the accompanying drawing is a diagram enabling a comparison of the stress-rupture properties of one of my stainless steels (steel A), with those of three steels falling outside the critical composition range.

As conducive to a, clearer understanding oi' 40 certain features of my invention it may be noted at this point that a, great variey of steels in the prior art, including a number of the austenitic stainless steels, tend to form a constituent at high temperatures called the sigma phase. This phase although hard and brittle at room temperatures is a non-magnetic intermetallic compound which I find to be weak and ductile at high temperatures. This phase slowly develops while the steel is heated, and when present in appreciable quantities (above about 2%) impairs creep strength and stress-rupture properties of the hot metal. 'I'he phase produces a high temperature weakening eiect which in extreme instances becomes quite pronounced. It will, therefore, be appreelated that more than small quantities of the phase Vare detrimental to the properties of high temperature steels, although these properties perhaps are only impaired and not entirely destroyed.

The austenitic chromium-nickel steels in general may be viewed as high temperature steels. Unfortunately, however, these steels are susceptible to creep and stress-rupture, the susceptibility at times being intensified by the hurtful effects of sigma phase. They nevertheless have a more favorable lattice structure for cohesion under high stress than do ferritic straightchromium stainless steels. This distinction alone, however, does not necessarily establish good high temperature properties.

As will be more fully understood from my copending applications identied above, I have previously developed certain austenitic stainless 0 steels and articles which have special high temperature utility. These steels, however, while possessing an unusual ability, so far as high temperature steels are concerned, of being readily hot worked, and numerous other characteristics including corrosion resistance and considerable resistance to creep and stress-rupture, are not collectively free of the impairing effects of sigma phase. At temperatures of about 1500 F., the stress-rupture and creep values are found to substantially suffer.

An outstanding object of my invention accordingly is the provision oi' creep resistant and stress-rupture resistant austenitic chromiumnickel stainless steel having substantial freedom from sigma. phase at high temperatures, particularly high temperature austenitic chromium-nickel stainless steel articles such as bolts and fasteners, internal combustion engine valves, gas or steam turbine rotors, buckets, nozzles, and a host of other articles suited for resisting mechanical stress under the high temperature conditions of use.

Referring now more particularly to the practice of my invention, I provide austenitic stainless steels and articles, which in having highly critical composition limits and by containing definite elements such as carbon, chromium, nickel, molybdenum, copper, titanium, columbium, and the remainder substantially iron, are for the intents and purposes free of sigma phase and have excellent high temperature properties including high resistance to creep and rupture. More specically, I provide stainless steels containing approximately 0.09% to 0.20% carbon, 15.75% to 16.75% chromium, from 13.25% to 14.75% nickel,

1.75% to 2.8% molybdenum, from 2.5% to 3.5% copper, 0.10% to 0.25% titanium, from 0.35% to 0.55% columbium, from 0.20% to 1.5% manganese, up to 0.70% silicon, preferably not more than 0.03% sulphur where hot working properties are desired, and the remainder substantially all iron. Where incidental amounts of phosphorus are present in the steel, these usually range between 0.01% and 0.04%. At times, I employ purposeful additions of phosphorus ranging up to 0.12% or more, with resulting benefits to the high temperature properties. Preferably,

the carbon, titanium and columbium contents are in such related amounts as to satisfy the formula:

a numerical value ranging from 5 to 10. By observing this formula. ratio more positive assurance is had against sigma. formation.

My stainless steels are wholly austenitic in structure. Ferrite, if present at all, is only in traces. 'I'his I lnd is essential to the required stress-rupture properties. Where appreciable amounts of ferrite are present, the stress-rupture values fall off; also the working qualities of the metal surfer.

The composition limits given in fact are considered to be in every sense critical since I nd that where they are departed from one or more of the desired qualities suffer. For example, with a lower carbon content the creep and stressrupture properties suier even at temperatures as low as 1200 F., and because sigma formation is promoted. Carbon in amount exceeding the high side of the range indicated adversely affects hot working properties and high temperature rupture properties. To like eiect, with a manganese content exceeding 1.50%, there is a loss in hot working properties. With any appreciable lowering of the chromium content below 15.75%, impaired scaling resistance in oxidizing atmospheres becomes noticeable at high temperatures. Chromium contents exceeding 16.75% markedly promote sigma phase formation. A nickel content below 13.25%, at the chromium level which I employ, encourages the development of sigma phase. Where the nickel content is above 14.75%, this causes the steel to be sluggish in response to heat treatment, and decreases the stress-rupture properties and creep strength of the metal. The element molybdenum, if less in quantity than hereinbefore indicated, produces a decrease in elevated temperature properties, while amounts in excess promote impairment by sigma phase. Any amounts of copper appreciably below the set limit of 2.5% also are detrimental to elevated temperature properties; amounts appreciably exceeding the upper limit of 3.5% adversely affect the hot working properties. The elements columbium and titanium in the quantities indicated hereinbefore given maximum high temperature strength and freedom from sigma phase.

In a preferred embodiment of my invention, the austenitic stainless steels which I provide more specifically contain about 0.09% to 0.20% carbon, 15.75% to 16.50% chromium, from 13.40% to 14.40 nickel, 2.25% to 2.75% molybdenum, from 2.5% to 3.5% copper, 0.10% to 0.25% titanium, 0.35% to 0.55% columbium, from 0.20% to 1.5% manganese, up to 0.70% silicon, up to about 0.03% sulphur, and the remainder substantially all iron. Incidental or purposeful amounts of phosphorus are present as already indicated in connection with the broader composition range of the alloy steel. The amounts oi. carbon, titanium and columbium advantageously are consistent with the formula ratio previously noted.

I sometimes employ a certain form of heat treatment for enhancing the high temperature load carrying capacity of my stainless alloy steels. In this treatment, I heat the steel to within a. solution temperature range of about 2050" F. to 2300 F., at which temperatures substantially all of the titanium and part of the columbium go into solution. The solubility of columbium increases in the higher side of the heating range. For reasons which will appear more fully hereinafter, I prefer to employ annealing temperatures of about 2200 F. to 2300 F.

With the titanium and part of the columbium in solution, I quench the steel as in air, oil, or water, conveniently to a temperature approaching that of the quenching medium. Then, for precipitation treatment, I heat the metal up to anywhere from 1200 to 1500 F., at which time columbium, titanium and copper too it is thought, precipitate in the matrix, the optimum temperature being about 1350 F. In this general treatment, a finely divided precipitate is critically dispersed in the metal lattice along the slipA planes. A portion of the precipitated titanium and columbium appears in intermetallic compounds. The titanium and columbium to some extent precipitate as carbides which increase in amount on the higher side of the precipitation treatment temperature range just noted. The copper comes out in ne form it is believed, or possibly asan intermetallic compound including titanium, columbium and nickel. No particular advantage seems to accrue in extending the time of precipitation treatment beyond iive hours, although up to fty or one hundred hours may be used without detriment. The treating time accordingly is not highly critical.

I quench the steel from the precipitation treated condition. The quenched metal has a fine grain structure, and is further characterized by enhanced load-carrying capacity in view of atomic slip interference developed by the precipitates. These precipitates remain uncoalesced and eective against creep and stress-rupture of the steels for extremely long periods of time in high temperature use o1' the steel. By the annealing, especially Within the preferred temperature range of 2200 F. to 2300 F., followed by the precipitation treatment, I substantially arrest any remaining tendency which the untreated steel may have toward sigma phase development.

Quite frequently, I use my stainless steels without rstsubjecting them to the combined annealing and precipitation treatment. The untreated steels are useful at temperatures mounting up to 1500 F. or more and have outstanding resistance to stress-rupture, creep, and are substantially free of the impairing effects of sigma phase. The combined heat treatment, however, I find aids as a supplemental protective measure against sigma formation particularly at extremely high temperatures of use.

As illustrative of the practice of my invention, I produce austenitic chromium-nickel stainless steel following the composition limits hereinbefore noted, the production conveniently being achieved in accordance with general stainless steel practices described in Patent 1,925,182 of Alexander L. Feild. By hot rolling or forging ingots of the steel, :for example from a temperature of 2250 F., I produce billets. IIfhese are then forged into roughly formed high temperature gas turbine parts or other articles intended for high temperature duty. Machining to final size is achieved as desired. Fabrication of certain parts as from sheet, strip, wire or the like, and by welding with the Oxy-acetylene torch, or electric arc means, employing welding rods preferably of approximately the same analysis as the metal being welded, is undertaken where desired. The finished articles are capable of withstanding temperatures `up to 1500 F. or more. They are strong in tension, compression and torsion at the high temperatures encountered as distinguished from excessive elongation, Weakness and prema-,

ture failure where sigma phase develops. Also,

in employed. The steels which I provide are especially reliable by reason of the substantial absence of sigma phase development, and accordingly are well suited for carrying load or resisting stress at high temperature. The steels in being amenable to annealing and precipitation heat treatment, thus within themselves oder afactor ofsafety against sigma formation over and above the excellent resistance in this respect afforded by the untreated metal.

Thus it will be seen that there is provided in this invention austenitic chromium-nickel stainless steel and products thereof in which the 'various objects noted, together with many thoroughly practical advantages are successfully achieved. It will be seen that the products are tough, strong and durable, corrosion-resistant and heat-resstant and are well adapted to withstand continuous high temperature duty over long periods of time and under the many variable conditions of actual practical use.

While there are lmany advantages attendant upon the inclusion of both of the elements titanium and columbium in my high temperature steels and products, and Iwhile the use of both elements is preferred, there are occasions where I modify the steels to the extent of using either titanium or columbium to the exclusion of the other, all further ingredients of the steel remain- The following tables are provided to illustrate 3o ing Substantially the seme in character and stress-rupture properties of one of my austenitic amount .as hereinbefoi-e desoribed Upon emstainless steels, this being identified as steel A in pioying the ingredient titanium under these. oir- Table I- Steels B, C and D' also identified are cumstances, I usually resort to amounts within outside the critical composition range of my the range of 020% to 050% so as to compensate steels, and are introduced to enable a comparison to some extent for the absence of oo1umb1umof stress-rupture properties. All of the steels These amounts are advantageously further reunder intermediate consideration were subjected strioted tooonsistenoy with the formule ratio; at like temperatures to my annealing and precipl itation treatment, and thereafter were given 2(%Ti)= stress-rupture tests at 1500 F. 40 C Table I.-Compositon a numerical value ranging from 5 t0 10. Consten o Cr Ni M0 ou T1 Cb Mn si s P `rs .1 0` 130 10.30 14.38 1. 7s 3.03 0.10 0.30 0.78 0.53 0. 015 0. 01s Bal. Bl 0.085 14. 26 16.71 4.31 4.59 0.41 0.49 0.53 0.012 0.000 BB1. 0.076 17.25 14.00 2.83 3.14 0.22 0.36 1.45 0.49 0.009 0.025 B81. DI 0.001 10.85 24.80 2.95 3.00 0.31 0.45 1.32 0.02 0.011 0.015 Bal.

l Steels outsidu the present critical range.

The results of the stress-rupture tests just menversely, in using columbium to the exclusion oi tioned are indicated with respect to several petitanium, the amounts of ycolumbium preferably riods of loading time in Table II. Curves showrange from about 0.60% to 0.90% and further are ing these and other stress-rupture values which advantageously held consistent with the formula may be expected with relation to time under ratio: stress, are to be found in the accompanying 7Gb drawing. It will be seen that the steel A, representing my steels, is considerably superior. 0

Table II.-Stress rupture test at 1500J F. at nlmelal alge ll'ling t0 o- 1The ,SM s. i. ooseessoprov e,we ero numo couml n ss (p1. riqumd t0 rupture steelm t1memdicaled.; bium grade, are amenable to the annealing and $00000 100 0000 10' 000 100, 000 precipitation heat treatments hereinbefore dehrs. hrs. hrs. hrs. scribed. Y As many possible embodiments may be made 11,000 12,200 8.900 0,500 of my invention and as many changes may be Elim; 1% made in the embodiment hereinbefore set forth, 1) 11500 y01500 51200 3500 it is to be understood that al1 matter described herein is to be interpreted as illustrative and not My austenitic chromium-nickel stainless steels especially where the sulphur content is low are capable of fabrication much on the order of the more commonly known austenitic steels, as by hot forming, yet have outstanding high temperature properties, this by virtue of the particular and highly critical combination of elements therefrom 2.5% to 3.5% copper, 0.10% to 0.25% titanium, 0.35% to 0.55% columbium, from 0.20% to 1.5% manganese, up to 0.70 silicon, and the remainder substantially all iron.

2. Hot workable austenitic stainless steel substantially free of sigma phase at elevated temperatures, said steel containing approximately 0.09% to 0.20% carbon, 15.75% to 16.75% chromium, from 13.25% to 14.75% nickel, 1.75% to 2.8% molybdenum, from 2.5% to 3.5% copper, 0.60% to 0.90% columbium, from 0.20% to 1.5% manganese, up to 0.70% silicon, up to 0.03% sulphur, and the remainder substantially all iron, and said carbon and columbium further being restricted to amounts consistent with the iormula a numerical value ranging from to l0.

3. Hot workable austenitic stainless steel substantially free of sigma phase at elevated temperatures, said steel containing approximately 0.09% to 0.20% carbon, 15.75% to 16.75% chromium, from 13.25% to 14.75% nickel, 1.75% to 2.8% molybdenum, from 2.5% to 3.5% copper, 0.20% to 0.50% titanium, from 0.20% to 1.5% manganese. up to 0.70 silicon, up to 0.03% sulphur, and the remainder substantially all iron, and said carbon and titanium further being restricted to amounts consistent with the formula:

a numerical value ranging from 5 to 10.

4. Heat treated austenitic stainless steel substantially free of sigma phase at elevated temperatures, containing approximately 0.09% to 0.20% carbon, 15.75% to 16.75% chromium, from 13.25% to 14.75% nickel, 1.75% to 2.8% molybdenum, from 2.5% to 3.5% copper, 0.10% to 0.25% titanium, 0.35% to 0.55% columbium, from 0.20% to 1.5% manganese, up to 0.70% silicon and the remainder substantially all iron; said steel being characterized by heating within solid solution temperature range of said copper, titanium and columbium, and by re-heating to within temperatures giving precipitation and critical dispersion of a iinely divided precipitate including titanium and columbium from solution.

5. Heat treated austenitic stainless steel substantially free oi' sigma phase at elevated temperatures, containing approximately 0.09% to 0.20% carbon, 15.75% to 16.75% chromium, from 13.25% to 14.75% nickel, 1.75% to 2.8% molybdenum, from 2.5% to 3.5% copper, one oi the elements 0.20% to 0.50% titanium and 0.60% to 0.90% columbium, from 0.20% to 1.5% manganese, up to 0.70% silicon and the remainder substantially all iron; said steel being characterized by heating within solution temperature range; and by re-heating to achieve precipitation from solution.

WILLIAM CHARLES CLARKE, JR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

1. AUSTENITIC STAINLESS STEEL SUBSTANTIALLY FREE OF SIGMA PHASE AT ELEVATED TEMPERATURES, SAID STEEL CONTAINING APPROXIMATELY 0.09% TO 0.20% CARBON, 15.75% TO 16.75% CHROMIUM, FROM 13.25% TO 14.75% NICKEL, 1.75% TO 2.8% MOLYBDENUM, FROM 2.5% TO 3.5% COPPER, 0.10% TO 0.25% TITANIUM, 0.35% TO 0.55% COLUMBIUM, FROM 0.20% TO 1.5% MANGANESE, UP TO 0.70 SILICON, AND THEN REMAINDER SUBSTANTIALLY ALL IRON. 